1. Field of the Invention
The present invention relates to an apparatus for detecting a heartbeat synchronous wave which is produced from a living body in synchronization with heartbeat of the living body.
2. Related Art Statement
There is known an apparatus for detecting a heartbeat synchronous wave produced from a living body in synchronization with heartbeat of the living body, through a probe which is adapted to be set on a body portion of the subject. For example, there is a pulse oxymeter for measuring a blood oxygen saturation of a subject, which includes (a) a first device for emitting toward a body portion of the subject a pair of lights having different wavelengths, (b) a second device for detecting the intensity of each of the lights transmitted through, or reflected from, the body portion and generating a photoelectric pulse wave signal representative of the detected intensities of the lights, and (c) a probe which is set on the body portion and which supports the first and second devices. The magnitude of the photoelectric pulse wave signal varies in synchronization with arterial pulsation (or heartbeat) of the subject, and the pulse oxymeter determines a blood oxygen saturation of the subject based on each pulse of the photoelectric pulse wave. In addition, there is a pulse wave detecting apparatus for detecting a pressure pulse wave produced from an arterial vessel of a subject in synchronization with heartbeat of the subject, through a probe which is set on a body portion of the subject and which supports a pressure sensor adapted to be pressed against an underlying arterial vessel to detect the pressure pulse wave. The pressure pulse wave can be used for determining a blood pressure of the subject.
However, the probe of each of the above-indicated apparatus may be moved relative to a body portion of a subject when a physical motion of the subject occurs. In such cases the heartbeat synchronous wave obtained by the apparatus contains a variation resulting from the relative movement. Meanwhile, in the event that a patient is undergoing a surgical operation, a heartbeat synchronous wave may contain a variation due to an "internal" cause such as use of a surgical knife or administration of a drug, in addition to the variation due to an "external" cause such as the above-indicated relative movement between the probe and the subject's body. Conventionally it has been considerably difficult for a doctor or a nurse to distinguish the variation due to the physical motion of the patient from the variation due to other causes, on the obtained heartbeat synchronous wave. Thus, the conventional apparatus suffer from the problem of providing an inappropriate heartbeat synchronous wave containing variations due to physical motions of a subject in addition to variations due to other causes, or the problem of providing blood oxygen saturation readings or blood pressure readings which are determined based on the inappropriate wave.